1. Field of the Invention
The present invention relates generally to the of heat transfer systems. More specifically, the present invention discloses a thermal management system using a capillary pumped loop in which the condenser tube is deformable and/or deployable.
2. Statement of the Problem
Management of thermal loads aboard space Vehicles presents a unique combination of problems. Equipment within the space vehicle generates excess heat which must be dissipated. This is typically accomplished by means of a working fluid which accepts heat generated within the space vehicle, and is then circulated through external radiator panels where the heat is rejected by radiative heat transfer to outer space. In the simplest conventional arrangement, the working fluid remains in one phase (i.e. liquid) and is circulated by a mechanical pump. However, this arrangement has disadvantages in terms of the added weight of the pump and liquid coolant; power requirements to drive the pump; relatively coarse temperature control of the electronics and other thermal energy sources within the spacecraft; large temperature drops from source to sink which adversely affect total spacecraft weight and volume; and increased difficulty of system integration due to the interdependence of loads (i.e. on/off, up/downstream, etc.).
Other considerations arise in providing for deployment of radiator panels after orbit is achieved. Conventional designs use various types of bellows or seals at the flexure joints connecting the radiator tubes with the remainder of the heat management system located within the space vehicle. These seals and bellows may be subject to leakage or failure, with potentially catastrophic results.
Heat pipes have the potential of addressing some of these shortcomings. A conventional heat pipe is a self-contained heat transfer device with no moving parts. Heat is transported from one end of the heat pipe to the other by evaporation and condensation of an internal working fluid. Substantially the entire length of the heat pipe is filled either with a wicking material or capillary passages. All motion of the working fluid is accomplished by capillary pumping in the heated zone. In traditional heat pipes, liquid and vapor flow in opposite directions within a common tube. Liquid flows opposite the vapor in a generally linear geometric arrangement. However, due to technical constraints associated with maintaining the integrity of the wicking material or capillaries between the ends of the heat pipe, a reliable design for a flexible heat pipe has not been achieved. In particular, prior efforts in achieving a flexible heat pipe have relied upon complex passages for both liquid and vapor flow, or flexible lines such as welded bellows joints with an internal screen tube. Perhaps more importantly, such designs must contend with the difficult fabrication of a transition wick at the joint, and imply the use of an arterial (composite wick) heat pipe which is inherently less reliable than a non-composite wick pipe and cannot be easily reprimed in-orbit and under load. Therefore, conventional heat pipes cannot be used in association with a deployable radiator panel.
Two-phase heat transfer systems, and in particular, capillary pumped loops have substantial advantages for space applications. In contrast to heat pipes, liquid and vapor flows are in separate tubes and evaporation and condensation occur in distinct components in a CPL. The concept of a capillary pumped loop ("CPL") was developed in the mid 1960's by F. J. Stenger at the NASA Lewis Research Center (F. J. Stenger, "Experimental Feasibility Study of Water-Filled Capillary-Pumped Heat Transfer Loops," NASA TM X-1310, NASA Lewis Research Center, Cleveland, Ohio, 1966). Development continued at the NASA Goddard Space Flight Center with construction of a number of CPL's beginning in the late 1970's. Several of these systems have been developed at the Goddard Space Flight Center and one has twice been flown on the space shuttle to demonstrate micro-gravity operation. The basic operation of a CPL involves pumping a working fluid through the heat transfer system with capillary forces developed in a wick material located inside the evaporator. A CPL has no moving parts and is self-controlling, in that the flow rate of working fluid through the evaporator will automatically change to match the thermal load. CPL's are ideal for managing heat loads in spacecraft where vibrations, such as those from a mechanical pump, are detrimental. In addition, CPL's offer high reliability due to the absence of moving parts. They offer automatic heat load sharing if a number of evaporators are used in parallel. Phase separation and flow distribution are automatically controlled since the flow rate through each evaporator is related directly to the rate of evaporation at the wicking surface inside. Consequently, adjacent evaporators can operate at significantly different heat input rates, but both will have only working fluid vapor at their exits.
U.S. Pat. No. 3,152,260 of Cummings discloses a solar power plant in which coolant is circulated through radiator panels 30 to dissipate excess heat. Each panel have flexible loops 34 connected to the panel conduits to circulate the coolant, as shown most clearly in FIGS. 4 and 5. These flexible loops act as springs which cause the panel sections 30 to deploy to their outstretched positions after the satellite has achieved orbit.
3. Solution To the Problem
None of the prior art references show a CPL having a deformable wickless condenser tube. This arrangement combines the thermodynamic advantages of a CPL with the physical advantages of flexible or deformable condenser tubes that enable radiator panels to be deployed without the need for bellows or seals.